The present invention is further detailed in Japanese Patent Application Nos. 5-82660 and 5-82661, both filed Mar. 16, 1993, for which the present application claims priority and the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention provides a process for molding a semi-rigid integral skin foam, wherein the semi-rigid integral skin foam comprises an integral skin region having a low degree of foaming and a core region having a high degree of foaming. The present invention further provides a polyurethane material for molding such a foam, which is suitable for reaction injection molding (RIM) of a molded product having an integral skin foam, such as a cover or pad for a steering wheel of a motor vehicle, an instrument panel, a console box lid, a glove box lid, a headrest, an armrest, or an air spoiler.
2. Description of Related Art
A semi-rigid polyurethane foam containing an integral skin region having a low degree of foaming on its surface and a core region having a high degree of foaming at its center is generally referred to as an integral skin foam (ISF). An ISF is produced from a long-chain polyol having a molecular weight of the order of thousands, such as 1,000-9,000, a low-molecular polyol having a molecular weight of the order of tens to hundreds such as 10-900, a catalyst, a pigment, freon as a foaming agent, and an isocyanate component. In normal operations, all the components, except the isocyanate component, are preliminarily mixed to give a polyol mixture. The polyol mixture is mixed with the isocyanate component by a mixing head in a prescribed ratio to give a polyurethane material. Finally, the polyurethane material is injected into a mold cavity made of a highly heat-conductive material, so that it expands in the mold cavity. The freons which are normally employed include CFC-11 (trichloromonofluoromethane) and HCFC-123 (2,2-dichloro-1,1,1-trifluoroethane). For example, about 16 parts by weight of HCFC-123 are used with 100 parts by weight of the polyol component for molding an ISF cover for a steering wheel.
Upon injection into the mold cavity, the polyol mixture and the isocyanate component undergo a urethane-forming reaction, which is exothermic enough to vaporize freon in the polyurethane material into a myriad of bubbles. The polyurethane material slowly expands and flows in the mold cavity, expelling air from a vent hole of the mold cavity.
At the center of the polyurethane material, the urethane-forming reaction and the vaporization of freon take place simultaneously, sustaining a myriad of freon bubbles. This results in the formation of a core having a high degree of foaming. The polyurethane material close to the wall surface of the mold cavity undergoes the urethane-forming reaction more slowly than that at the center, because the reaction heat dissipates through the wall of the mold cavity. One skilled in the art will appreciate that the slower reaction is characterized by a lower reaction heat and, consequently, a reduced number of bubbles will be formed from a decreased vaporization of freon. The bubbles are compressed or collapsed by the internal foaming pressure. The result is the formation of an integral skin having a low degree of foaming.
The above-described conventional method for producing an ISF by the aid of freon poses the following problems: freon release to the atmosphere from any industrial or commercial process is recognized as a global problem and reductions in its use are being mandated by many countries; formation of the integral skin by collapsing bubbles at the surface section by the internal foaming pressure allows minute bubbles to inevitably remain in the skin which ultimately impair the appearance of the final foamed product; and overpacking and subsequent discharge of the polyurethane material from a vent is necessary to increase the internal foaming pressure and to promote the mold filling. Ultimately this leads to a great loss of material.
An effective way of increasing the production efficiency of producing urethane foam articles is to increase the reaction rate of the polyurethane material, thereby reducing the cure time. This object can be achieved by increasing the amount of the catalyst or raising the temperature of the polyurethane material before injection. The latter is preferable to the former, which leads to an increase in production cost. It has, however, been actually impossible to adopt the latter method when freon is used as a foaming agent, since its vaporization or a rise in tank pressure is likely to occur immediately after the injection of the polyurethane material. Moreover, the use of freon in a large amount can delay the curing of the product of the urethane-forming reaction and thereby prolong the cure time, as the heat of the urethane-forming reaction is dissipated due to vaporization of the freon. By eliminating freon in the urethane-forming reaction, the present invention provides greater alternatives in this regard.
Further, freon foamed processes which include a core in the mold cavity, such as is common in the production of steering wheels, suffer from turbulence induced defects such as pinholes, voids and short shot, due to entrapped air. This is particularly a problem in the production of steering wheels, in which case a long ring core is placed in the mold cavity. Thus, it is necessary to select an adequate place for gating to minimize turbulence and to form several vents to release entrapped air.
Alternative methods for producing polyurethane foams using reduced pressure are disclosed in the following Japanese Patents, however, these patents do not necessarily disclose or suggest the object, constitution and effect of the present invention.
Japanese Patent Laid-open Nos. 55-63237 and 55-63238 disclose a method for causing a polyurethane material to uniformly expand to fill a mold cavity completely, by evacuating the mold cavity through thin grooves formed in the walls of the mold cavity. However, these patents presumably teach the use of standard foaming agents as only conventional compositions are disclosed.
Japanese Patent Laid-open No. 56-111648 discloses a method for foaming a polyurethane material in an atmosphere of reduced pressure. The patentee claims that this method provides for a reduction in the amount of foaming agent, such as freon, required. However, this method employs freon as conventionally used. This patent does not provide for the advantageous elimination of freon, as in the present invention.
Japanese Patent Laid-open No. 62-164709 discloses a method for producing a low-density polyurethane foam by expanding a polyurethane material which contains water as a foaming agent in an atmosphere of reduced pressure. Unlike the present invention, this method is not intended to produce an ISF. The formation of the skin is not desirable. Japanese Patent Laid-open No. 63-268624 discloses a method for producing a polyurethane foam by causing a polyurethane material to capture nitrogen gas as much as 2-30 vol %, and expanding the polyurethane material in an atmosphere under reduced pressure. This method is not taught to be useful to produce an ISF.
Japanese Patent Publication No. 64-5528 discloses a method for producing a polyurethane foam by injecting a rigid polyurethane material into an evacuated mold cavity at a packing ratio of about 150-450%, and subsequently expanding the polyurethane material, however, the polyurethane material contains a foaming agent, such as freon.